1. Field of the Invention
This invention relates to an organic thin-film EL device, particularly to a cathode of an organic thin-film EL device having light emitting picture elements arranged in the matrix form.
2. Prior Art
An organic thin-film EL device makes use of the phenomenon that holes injected from an anode and electrons injected from a cathode recombine in an emitter layer and emits light after the excitation state. Although various device structures have been studied according to the properties of the light emitting material of the emitter layer, devices can be fabricated principally by sandwiching between the anode and cathode an organic emitter layer which emits strong fluorescence. For the heightening of luminous efficiency and stable operation, it is considered effective to dispose a charge transport layer such as hole injection transport layer or electron injection transport layer or to carry out doping of guest molecules into an organic emitter layer. Furthermore, electrode materials, particularly, cathode materials are under investigation with a view to improving the luminous efficiency or lifetime properties.
As a cathode, it is effective to employ an alloy metal having a work function as low as possible. It is reported that from an organic thin-film EL device using, among various alloy metals, lithium-containing aluminum having a low work function, light emission of high efficiency, high luminance and long lifetime is available.
For example, it is reported in Japanese Patent Application Laid-Open No. 165771/1985 that in an organic thin film having an emitter layer, which is contiguous to a cathode, made of anthracene, light emission with high efficiency is available when an aluminum-lithium alloy or magnesium-lithium alloy is used as a cathode. It is also reported that the optimum content of lithium in the alloy is 1 to 99 wt. %, preferably 10 to 50 wt. %.
On page 6 of Japanese Patent Application Laid-Open No. 212287/1992, it is reported that light emission with higher luminance can be attained using tris(8-quinolinol) aluminum (which will hereinafter be abbreviated as xe2x80x9cAlqxe2x80x9d) as an electron transport emitter layer contiguous to a cathode and as the cathode, an alloy containing at least 6 mole % of an alkali metal (for example, an aluminum alloy containing 28 mole % of lithium or magnesium-lithium alloy).
In Japanese Patent Application Laid-Open No. 121172/1993, it is reported that light emission with high efficiency and long lifetime and EL emission with high environmental stability can be attained in the case where a lithium-aluminum alloy is used as a cathode contiguous to an organic fluorescent thin film made of Alq and this alloy has a lithium concentration of 0.01 to 0.1 wt. %.
Thus, in an organic thin-film EL device having a cathode made of a lithium-containing alloy, it is considerably important to select a suitable concentration range of lithium to be incorporated in the cathode in accordance with an organic material adjacent to the cathode with a view to imparting the organic thin-film EL device with excellent efficiency and lifetime properties and high environmental resistance.
An organic thin-film EL device which uses a cathode material containing lithium within a certain composition range exhibits light emission of relatively high luminance and high efficiency.
In the case where Alq is used as an emitter layer, since Alq emits a green light, light emission on the shorter-wavelength side than it (for example, blue light emission) is not available even by doping into the emitter layer made of Alq. In the case where Alq is used as an electron transport layer, the use of an emitter layer having a large excitation energy or charge injection level (ex. blue-light emitting material) does not permit the sufficient closure of excited electrons or charges, which makes it difficult to attain sufficient luminous efficiency or luminance. The compound of the formula (I) which will be described later is used in this invention in view of such defects.
The above-described composition range of lithium is used only when an organic thin-film layer adjacent to a cathode is a specific compound (ex. Alq). If an organic thin film made of the compound as represented by the below-described formula (I) is used as an organic thin-film layer adjacent to a cathode, lithium of the above-described composition range does not bring about sufficient luminous efficiency or increases a luminance lowering rate while the resulting element is driven. Such disadvantages occur because the electron injection level of the organic thin film varies depending on the organic material to be employed so that the injection efficiency of electrons is presumed to be insufficient in a known lithium concentration range; and because adhesion between the organic thin film and cathode is weak in the known lithium concentration range, which lowers the electron injection efficiency and makes it difficult to attain stable light emission.
An object of the present invention is therefore to provide an organic thin-film EL device which has high efficiency and high luminance, is free from a substantial deterioration in the luminance and has a high yield when producing the same.
The present inventors have found that an organic thin-film EL device having high luminous efficiency (luminance per current density) and has long lifetime can be obtained using an aluminum alloy containing lithium in a concentration range of 0.05 to 1.5 wt. % as a cathode for the organic thin-film EL device wherein an organic thin film layer adjacent to the cathode contains an organic compound represented by the below-described formula (I).
In the present invention, there is thus provided an organic thin-film EL device of a charge injection type which has an organic thin film layer adjacent to the surface of a cathode opposite to an anode, said organic thin film layer containing an organic compound represented by the following formula (I): 
wherein R1 to R6 each independently represents a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group or a cyano group; L represents a group xe2x80x94OR7 in which R7 representing an alkyl group, a cycloalkyl group, an aromatic group which may contain a nitrogen atom, an aromatic group having a bonding group composed of a metal atom or an oxygen atom, or a ligand of an oxinoid compound having said bonding group; M represents a metal atom; and n stands for an integer of 1 or 2, said cathode comprising aluminum as a main component and lithium in an amount of 0.05 to 1.5 wt. %.
In another aspect of the present invention, there is also provided an organic thin-film EL device of a charge injection type having an organic thin-film layer adjacent to the surface of a cathode opposite to an anode, said organic thin-film layer containing an organic compound represented by the above-described formula (I) and said cathode comprising magnesium as a main component and lithium in an amount of 0.03 to 1.7 wt. %.
The present invention makes it possible to heighten the basic properties of the device such as luminous efficiency and lifetime.
In addition, lithium is contained in the aluminum alloy cathode of the present invention in a concentration range as wide as from 0.05 to 1.5 wt. % so that even a slight difference in the lithium composition does not have a large influence on the properties of the device, which makes it possible to fabricate an organic thin-film EL device in a high yield, more specifically, to widen the area of the device without losing uniform luminance and efficiency. Furthermore, neither corrosion nor black spot appears on the cathode easily.
On the other hand, lithium is contained in the magnesium alloy cathode of the present invention in a concentration range as wide as from 0.03 to 1.7 wt. % so that similar advantages as described above can be obtained and neither corrosion nor black spot occurs easily on the cathode.